Abdel Basset Ali al-Megrahi, the former Libyan intelligence officer convicted of murdering 270 people when a bomb he planted on Pan-Am Flight 103 blew up over Lockerbie, Scotland, on Dec. 21, 1988, has been released from prison after serving eight years of a life sentence.

Making the announcement on Thursday, Justice Secretary of Scotland Kenny MacAskill said that Al-Megrahi was released on compassionate grounds because he has terminal prostate cancer and is unlikely to live past the next three months. A Libyan jet met Al-Megrahi at Scotland's Glasgow Airport to take him back to Tripoli, where he was greeted by hundreds of people, many of them waving flags. Al-Megrahi was met by Colonel Muammar Gaddafi's son, Seif al-Islam Gaddafi, who was quoted as saying, "I would like to thank the Scottish government for its courageous decision and understanding of a special human situation." (See pictures of Lockerbie 20 Years On.)

The release drew criticism from U.S. President Barack Obama's spokesman and many of the American relatives of those on board the flight, as well as investigators who prepared the evidence that led to Al-Megrahi's conviction. However, some British relatives of the Lockerbie victims and the legal expert who helped design the trial that convicted Al-Megrahi expressed support for the decision.

In a prepared statement, MacAskill said that compassion was "a defining characteristic of Scottish people" and that "the perpetuation of an atrocity cannot and should not be a basis for losing sight of who we are ... Mr. Al-Megrahi now faces a sentence imposed by a higher power. It's one that no court ... could revoke or overrule. It is terminal, final and irrevocable. He is going to die."

Al-Megrahi was convicted in a specially convened Scottish court at Camp Zeist in The Netherlands in 2001. Prosecutors argued that he placed a bomb, hidden in boom box inside a suitcase, on a flight from Malta to Frankfurt, Germany. From there, the bomb was transferred onto the Pan Am plane that went first to London's Heathrow Airport and then took off for New York City. The bomb exploded as the plane flew over Scotland, causing it to crash on to the town of Lockerbie near the Scottish border. Another man  Al-Amin Khalifa Fahima  was also tried for his involvement in the bombing but was acquitted. (Read: "Re-Opening the Lockerbie Tragedy.")

In 2003, Libya formally accepted responsibility for the Lockerbie bombing, paying $2.7 billion in compensation to relatives of the 259 people on board and 11 town residents who died in the attack. Since then, however, Libyan officials have denied culpability and suggested that the payout was made as part of its recent efforts to normalize relations with the West.

In a statement released after Al-Megrahi departed Glasgow, the convicted bomber expressed sympathy for the relatives of the victims but reiterated his claims of innocence. "This horrible ordeal is not ended by my return to Libya. Perhaps the only liberation for me will be death."

Speaking to TIME from her home in New Jersey, Susan Cohen, whose daughter Theodora would have been 41 next month had she not died on that Pan Am flight, said she believed MacAskill's decision was the result of a behind-the-scenes deal with Libya and that Lockerbie relatives were "the neglected victims of terrorism." She said that "the last tiny shred of justice we had for the death of my daughter is gone. There isn't even a living person in prison to show there was a Lockerbie."

Richard Marquise, who was the FBI special agent in charge of the U.S. investigation into the bombing, calls the release "very disappointing." In an unusual move, Marquise and his counterpart in Scotland, Stuart Henderson, the retired senior investigating officer for Lockerbie, had written to MacAskill to argue against Al-Megrahi's release, reiterating their belief that the evidence gathered against him was compelling. U.S. Secretary of State Hillary Clinton and seven senators, including Edward Kennedy and John Kerry, also wrote protest letters to MacAskill. After MacAskill's announcement, the White House put out a statement saying it "deeply regrets" Al-Megrahi's release. (See pictures of Hillary Clinton.)

In his announcement, MacAskill said his decision was not influenced by questions of whether Al-Megrahi's conviction was legitimate. However, he also said he felt that "there remain concerns [around] some of the wider issues of the Lockerbie atrocity. There are questions to be asked and answered." Doubt in Al-Megrahi's guilt is relatively widespread in Britain, even among legal experts, close observers of the trial and the families of some of the victims. Robert Black, a professor emeritus of Scots Law at Edinburgh University and one of the legal architects of the Camp Zeist trial, tells TIME that he is relieved by Al-Megrahi's release. "Al-Megrahi should never have been convicted in the first place," he says. "It's totally inexplicable that a court could have felt the evidence against him justified a guilty verdict."

Many of those who aren't convinced of Al-Megrahi's guilt say their greatest regret is that he abandoned the appeal to overturn his conviction last week, presumably to clear the way for his release. Juval Aviv, a private investigator who was hired by Pan Am's insurance company to investigate the bombing, tells TIME that "as time has passed, flaws in the trial and in the evidence used to convict Mr. Al-Megrahi have been revealed. A new trial could have shone a light on a lot of things that were buried in the rush to convict him." (Read: "Terrorism Solving the Lockerbie Case.")

Retired FBI agent Marquise believes that Al-Megrahi's release leaves the search for justice incomplete for a different reason. He feels frustrated that Al-Megrahi remains the only man convicted of a crime that was almost certainly not the work of one person. "He's got a lot of questions to answer, but now he's gone he's unlikely to do so," Marquise says. "The case is now closed, I'm afraid. And that's very disappointing."